1. Field of the Invention
The present invention relates to a method utilized in a wireless communication system and a communication device thereof, and more particularly, to a method of handling measurement reporting in a wireless communication system and a related communication device.
2. Description of the Prior Art
A Universal Mobile Telecommunications System (UMTS) is the 3rd generation mobile communication technology. A Universal terrestrial radio access network (UTRAN) includes a core network, which is composed of a plurality of Node Bs and a plurality of base station controllers (or radio network controllers, RNCs).
For communication quality maintenance, user equipments (UEs) may perform a measurement, to measure strength or signal noise ratio (SNR) of wireless signals. The UTRAN first has to send a measurement control message to a UE before the UE performs the measurement. The measurement control message includes a field of “measurement type”, which defines the measurement type (e.g. intra-frequency measurement) that the UTRAN request the UE to perform.
On the other hand, a measurement result is sent from the UE to the UTRAN by a measurement report message. Note that, with a field of “measurement report criteria” of the measurement control message, the UTRAN indicates which events trigger measurement report to the UE. For example, the UE may report the measurement result when a measured signal strength or SNR at a common pilot channel (CPICH) of a cell not included in an active set is greater than a CPICH of a cell included in the active set. The above condition is defined as a 1c type event and a condition for triggering the 1c type event is complied with a number of the cell in the active set is greater than or equal to a cell replacement activation threshold, wherein the cell replacement activation threshold is a minimum number of cells allowed in the active set, and can be configured with the measurement control message. The detailed description can be referred to 3GPP TS 25.331, so it is not given herein. However, the 1c type event may cause cells with poor signal strength or the SNR to be replaced.
Note that, if the UTRAN does not update the active set (e.g. the network does not send an active set update message or the UE does not receive the active set update message) after the UE sends the measurement report message, the UE may send a new measurement report message after a predefined interval (hereinafter called time-to trigger (TTT)). On the other hand, the UE may periodically send the measurement report message to the network according to a periodic reporting interval. For example, the network sets the periodic reporting interval to be 5 seconds, thereby the UE may send measurement report message to the network every 5 seconds.
However, in the current 3rd generation partnership project (3GPP) specification, when the network receiving the measurement report message triggered by the 1c type event, the network may indicate the UE to remove cells with good signal strength or SNR in the active set, causing a call drop and affecting a experience of a user, the detailed reasons is as following.
Please refer to FIG. 1, which is a schematic diagram of a conventional measurement reporting. In FIG. 1, the UE sends a first measurement report message (triggered by the 1c type event) to the network, to trigger a cell replacement procedure. The first measurement report message notifies the network to replace a cell B or C with a cell D. Assuming that the signal quality of the cell C is the weakest cell in the active set ASET, and thereby, the intention of the UE is to replace the cell C with the cell D.
According to the 3GPP specification, when the network handling the first measurement report message but a active set update message in response to the first measurement report message has not been sent to the UE, for example, the network does not response to all measurement report messages (e.g. the network considers that the communication quality of the cell does not need to be replaced), the UE may send a second measurement report message (triggered by the 1c type event) to the network actively after the TTT/periodic reporting interval, to trigger another cell replacement procedure, wherein the second measurement report message notifies the network to replace cell B or C with a cell E.
Note that, before the network sends the active set update message in response to the first measurement report message and the UE has not receive the active set update message, the UE may send the second measurement report message (reasons can be described as above) to the network. Since the network has already updated the cell information in the active set ASET, i.e. the network identifies that the cell C has been replaced with the cell D, when the network handling the second measurement report message (notifying to replace cell B or C with the cell E), the cell C has not included in the active set ASET of the network. Thus, the network may replace the cell B in the active set ASET with the cell E, causing that the cell (e.g. the cell B) with good communication quality is replaced. In short, since the cell information in the active set ASET has been updated by the network, the second measurement report message may cause the network unnecessarily replaces/removes the cell, impacting call service/quality. Besides, the cells D and E may have good signal quality/strength only in a short period, and thereby, when the UE leaves a service area of a cell A, the radio links between the UE and the cells A, D and E in the active set ASET may be released, causing the call drop.
As can be seen, the network may face a race condition with the two measurement report messages, such as the first measurement report message notifying the network to replace the cell B or C with the cell D, and the second measurement report message notifying the network to replace the cell B or C with the cell E. In this situation, the cell replacement procedure may cause a call drop since the cell C has been replaced with the cell D. On the other hand, the network in error assumes that the cells indicated in the measurement report message for the cell replacement are all in the active set. However, since the active set has been updated by the network, the cells indicated by the measurement report message may not include in the active set, causing the communication drop.